Traveling wave tube



Oct. 4, 1960 R. KOMPFNER ETAL 2,955,223

TRAVELING WAVE TUBE Filed Sept. 12, 1956 2 Sheets-Sheet 1 FIG.

SIG/VAL sou/ea: 0 COAX/AL LINE TERM/NA non UTILIZATION CIRCUIT FIG. 2B

FIG. 3A FIG. 3B

R. KOMPF/VER lNl/ENTO/PS G E QUA AT TORNEV Oct. 4, 1960 R. KOMPFNER ETAL2,955,223

TRAVELING WAVE TUBE Filed Sept. 12, 1956 2 Sheets=Sheet 2 FIG. 4A FIG.4B

R. KOMPFNER 'WENTOPS c. E QUATE A T TORNE V United States PatentTRAVELING WAVE TUBE Rudolf Komptner, Far Hills, and Calvin F. Quate,Berkeley Heights, NJ., assignors to Bell Telephone Laboratories,Incorporated, New York, N.Y., a corporation of New York Filed Sept. 12,1956, Ser. No. 609,483

15 Claims. (Cl. 315-35) This invention relates to a traveling wave tubeand, more particularly, to a traveling wave tube of the spatial harmonictype.

The present application is a continuation-in-part of an earlierapplication Serial No. 572,322, now abandoned, filed March 19, 1956.

In a traveling wave tube, an electromagnetic Wave is propagated along aspecially designed wave transmission line and an electron beam isprojected along a path in coupling proximity with the transmission linefor amplifying the propagating wave. As the beam passes in couplingproximity to the propagating wave, there results a bunching of the beamelectrons and a net deceleration of these electrons. This decelerationrepresents a loss in kinetic energy of the beam and a consequent gain inelectromagnetic energy of the field of the propagating wave. Thus thewave becomes amplified.

In order to achieve this energy transfer to the field of the propagatingwave, it is necessary that the speed of the electron beam beapproximately synchronous either with the fundamental of the propagatingwave or with a spatial harmonic component thereof. The present invention is concerned with the latter case where the speed of the beam issynchronous with a spatial harmonic component of the propagating waveand hence beam interaction is had with such component. Since this typeof interaction finds more use in traveling wave tubes of the so-calledbackward wave type than in forward wave type, the present invention willbe discussed with particular reference to the tubes of the former type,although it is to be understood that we do not intend to limit ourinvention to backward wave type tubes.

In backward wave operation, a wave propagating in one direction alongthe transmisison line interacts with a beam projected in the oppositedirection, the interaction occurring between the beamand a negativespatial harmonic component of the wave, as explained in an articleentitled A Spatial Harmonic Traveling-Wave Amplifier for Six MillimeterWavelength, by S. Millman, appearing in the Proceedings of the I.R.E.,vol. 39, page 1035, September 1951. Backward wave operation is useful athigh frequencies in both amplifiers and oscillators, particularly thelatter.

In one form of backward wave tube, the transmission line comprises aconductor wound to form a helix and the electron beam is projected alongthe axis of the helix. The conductor is advantageously a flat tape-likemember. Such a transmission line is particularly desirable because ofits structural simplicity and broad band characteristics.Disadvantageously, however, no one has priorly succeeded insimultaneously achieving efiicient operation and maximum gain with sucha transmission line when using it in conjunction with-a solid electronbeam. The reason for this will be explained hereinafter but, brieflyspeaking, it is because when the helix is dimensioned for maximuminteraction impedance, and consequently maximum gain, the spatialharmonic fields of a wave propagating the'realong are confined close tothe helix and subcated electrons of the beam involved the use of ahollow or annular beam. This eliminated the electrons which failed toachieve interaction with the spatial harmonic fields proximate thehelix, with a consequent saving in power. However, the use of an annularbeam is often undesirable since the amount of current containable insuch a beam is obviously substantially less than in a solid beam and,further, difliculties arise in generating and focusing such a beam.

Accordingly, an object of the present invention is to achieve high gainand high elficiency in a traveling wave tube of the spatial harmonictype which utilizes a solid electron beam in connection with ahelix-like transmission line.

To these ends, a feature of the present invention is an arrangement ofone or more conductive tapes wound in a helix-like manner, each of whichis designed to provide both high interaction impedance between spatialharmonic field components and the beam and the estab lishment of finitespatial harmonic fields over the entire beam cross section.

In certain illustrative embodiments of the present invention for use asa backward wave amplifier or oscillator, a solid electron beam ofsubstantially circular cross section is projected along an extended pathand a helixlike member forming a wave propagating circuit is positionedalong a substantial portion of that path. For securing maximum gain fora backward traveling wave,

the distance between corresponding points on successive turns of thehelix-like member measured around the periphery of the helix, that is,the length of one complete turn, is adjusted to be approximately onehalf of a free space wavelength. Additionally, for establishing finitespatial harmonic field components over the entire beam cross sectionwhile maintaining maximum gain, each turn of the member has a firstportion contiguous to the beam path and a second portion more remotetherefrom and the contiguous portion of each turn lies along an arc of acircle whose circumference is approximately 'equal to 15k where is thewavelength of a wave propagating along the helix-like member.

In an alternative embodiment for use as an amplifier or oscillator, thehelix-like conductive member is replaced by two such members each ofwhich propagates a wave in interacting relation with the beam and whichtogether are likewise dimensioned to secure maximum gain and finitespatial harmonic field components over the entire beam cross section.

The invention will be explained in greater detail in the followingdescription taken in connection with the accompanying drawing, in which:t

Fig. 1 is a longitudinal sectional view of a traveling wave tube of thebackward wave type whose transmission line is shown in phantom;

Figs. 2A and 2B are perspective and end views, respectively, of atransmission line which comprises a single conductive tape wound to forma helix as used in tubes known in the prior art; 7 v

Figs. 3A and 3B are perspective and. end views,- respectively, of atransmission line comprising a single con-' 0 the principles of thepresent invention;

Figs. 4A and 4B are perspective and end views, respec tively, of atransmission line in accordance with another embodiment of' thisinvention which comprises a single conductive tape wound in a helix-likefashion;

Figs. 5A to SC show the steps of a technique for fabricating thestructure of Figs. 4A and 4B; and

1 and 6B- areperspective and endlviews', respectively, of: a.transmission line comprising two conductive tapes. wound'in a helix-likefashion, inaccordance with still another illustrative embodiment of thisinvention.

Referring now more: particularly to the drawing, Fig. 1' shows atraveling wave tube 10- comprising an evacuated envelope '11, typicallyof glass or nonmagnetic metal such as copper, enclosing an. electron gun12 for forming a solid electron. beam and projecting the. beam alonganextended path'toward collector 13. The: electron gun as. shownschematically includes a. cathode 15, heater 16, beamv forming electrode1.7, and accelerating anode E8; In. practice, these elements aremaintained in place by suitable supporting members, and there areprovided leadirr conductors from suitable voltage sources formaintaining the various elements at appropriate poten-- tials. Inparticular, the cathode is maintained at a potential slightly positivewith respect to the beam forming electrode and appreciablynegative withrespect to the accelerating anode so that the beam will be. projectedtoward the collector which is also biased positively with respect to thecathode. Additionally, permanent magnets 21 are provided. forestablishing a magnetic field whose fiux lines extend along the lengthof the beam path tor focusing the beam. These magnets may be replaced bya solenoid or other suitable apparatus for focusing, the beam along itslength.

, A transmission line 22,. depicted in Fig. 1' as a pair of dot-dashlines, is positioned to surround the beam'along a. major portion of itspath and is preferably arranged coaxi-ally with the beam axis. The lineis maintained at a suitable positive DC. potential. with respect to thecathode; Such potential fixes'the velocity of the electrons in theirflow past the line. Coaxial connectors 23 and 24 are provided at theupstream and downstream ends, respectively, of the transmission line fortransferring wave energy between the line and various external circuits.

For operation as' an amplifier of the backward wave type, Wave energyis' supplied from signal. source 25 and passes by way of coaxial line 24to transmission line 22. The wave energy thus supplied is amplified inpassing from right to left along the transmission line in a directionopposite to that of electron flow, in accord ance with the principles ofbackward wave operation wherein interaction occurs between the: electronbeam and a negative spatial harmonic of the wave, and finally passes byway of coaxial line 23 to utilization circuit 26..

For operation as an oscillator of the backward wave type, the signalsource 25 is replaced by a coaxial line termination'for making thedownstream end of the line substantially refiectionless and the electronbeam intensity is increased to a value sur'iicient to initiateoscillations. In this case, wave manifestations existing as noiseon theelectron beam initiate a Wave along transmission line '22. Additionalinteraction between the beam and a negative spatial harmonic componentof the initiated wave causes such wave to be amplified. Thewavethus-amplified is transferred by way of coaxial line 23 to utilizationcircuit. 26. The oscillator in efiect constitutes a closed loop formedby the electron beam which. servesv as a wave path to propagate waveenergy from left to right along the tube and transmission line 22' whichserves to propagate wave energy from right to left along the tube.

Transmission line 22 in the past has sometimes taken the form of aconductive tape wound to "form a simple helix, as shown in Figs.'2A and2B. With such a transmission line as the interaction circuit of abackward. wave device, it is found that efiicient operation cannot beachieved with a solid electron beam. The reason for this, viewed from aphysical standpoint, is that when 4 the helix is dimensioned for maximumgain the field of the spatial harmonic components of a wave propagatingtherealong is closely confined to the helix; substantially none of thisfield exists near the helix axis. Thus with the helix so dimensionedonly the periphery of a solid beam projected along the helix axis wouldpass through the high field region proximate the helix. Moreover, it isnot practicable, even if possible, toutilize a beam whose radius is asgreat as the inside radius of the helix. The outer electrons of such abeam would tend to strike the helix, causing damage thereto and creatingan intolerable amount of noise on the beam with a consequent distortionof the amplified wave. A practical limit on the radius of the beam isfound to be 0.821 where a is the efiective inner radius of thesurrounding interaction circuit, in this case the radius of the helix22A. Thus it can be appreciated that even the outer electrons of such abeam are somewhat removed from the helix. With a. beam having a radiusof 0;8a or less, a finite spatial harmonic field can be achieved at theaxis of the beam by dimensioning the helix so that 'yizequals 1.5, where"y is the propagation constant of the helix given approximately by theexpression where v A is. the wavelength infree space,

it is the velocity of propagation along the helix, and.

c is the velocity of the propagation in free space.

The requirement that we: equal 1.5 is acondition advantageously, in thesimple helix 22A it is not possible to satisfy this condition while atthe same time maintaining the distance between corresponding points onsuccessive turns of the helix measured around the periphcry of the helixequal to .5), where 7\- is a'f'ree space wavelength of the signal waveto be amplified, this latter condition being desirable for maximuminteraction impedance for the negative spatial harmonic components ofthe signal Wave, and hence for maximum gain.

However; both of these conditions may besatisfied, and hence maximumgain may be achieved and a finite spatial harmonic field establishedover the entire beam cross section in the helix-like transmissioncircuits in accordance with our invention.

The first of these helix-like transmission circuits 22B, shown inFigs.3A and 313, comprises a succession of turns of a conductive tape each ofwhich has an inner portion 32 which is positioned contiguous to the beampath along the axis of the helix and an outer portion 33 more remotefrom the beam path. The inner portions 32 effectively define the innerradius of circuit 2213- since The inner portions 32 effectively form asuccession of ring-like conductive members, each of which is notacomplete ring but includes an interruption, and these members areinterconnected by the outer portions 33' to form a continuous waveguiding structure.

Additionally, each of the inner potions 32 of the turns of thehelix-like member is made substantially pitchless whereby the electricfield of a wave passing along the circuit 22B is advantageouslysubstantially symmetric about the axis of a solid beam. Further, bysuitably choosing the pitch angle, the radius, and the length of theouter portions 33 which are not in interacting relation with theelectron beam, the distance between corresponding points on portions 32of successive turns measured around the periphery of the helix, that is,the length of one complete turn, can readily be made to equal a halffree space wavelength without disturbing the prescribed condition formaximum etficiency. Thus both the condition for achieving a finite fieldover the entire beam cross section and the condition for maximum gainmay be simultaneously satisfied. Both of the conditions cansimultaneously be satisfied in the structure 22B since, unlike helix22A, the inner radius a of the circuit and the distance betweencorresponding points on successive helixlike turns may be adjustedindependently. Contrariwise in the prior art helix 22A, these parametersare directly related, so choosing one fixes the other.

Another helix-like transmission circuit in accordance with our inventionis shown in Figs. 4A and 4B. This circuit 22C is substantiallykeyhole-shaped in cross section. As the circuit of Figs. 3A and 3B, itcomprises a succession of turns of a conductive tape each of which has afirst portion 42 positioned contiguous to the beam path and a secondportion 43 more remote fromthe beam path. In this circuit maximumefiiciency is achieved by dimensioning the contiguous portion 42 of eachhelix-like turn to lie along an arc, advantageously subtending an anglegreater than 300, of a circle whose circumference is approximately 15kor alternatively stated, to have dimension a of Fig. 4B approximatelyequal to Additionally, maximum gain is achieved by suitablychoosing thepitch angle and length of remote portions 43 to make the distancebetween corresponding points on contiguous portions 42 measured aroundthe periphery of the helix approximately equal to one half a free spacewavelength. This latter condition is readily achieved without disturbingthe prescribed condition for maxi mum efiiciency and so, as in thestructure of Figs. 3A and 3B, maximum efliciency and maximum gain arerealized simultaneously.

For maintaining the field of a propagating wave symmetric about the beampath through circuit 22C, the contiguous portions 42 of each helix-liketurn of the circuit are made substantially pitchless as shown in Fig.4A. In such an arrangement the remote portions are suitably pitched tojoin these pitchless portions in a man ner to achieve the dimensionsdescribed for maximum efficiency and gain. However, it is found that thecircuit is more readily fabricated if both the contiguous and remoteportions of each turn are similarly pitched. Additionally, it appearsthat the advantage obtained by this case in fabrication more thancompensates for the disadvantage of the slight field asymmetryoccasioned by virtue of the pitch of the contiguous portions. Thefabricating technique employed is to wind a simple helix on a suitablemandrel to have a cross-sectional shape which is substantiallyrectangular but has curved edges. Such a helix 51 is shown in Fig. 5Awound on mandrel 52. Various known coil winding machines may be employedfor winding the helix. Then a second mandrel 53 is positioned to extendlongitudinally within the helix, as shown in Fig. 5B, and pressure isapplied at one end of the two long sides of the rectangularlycross-sectional helix as shown by arrows 54. This results in the circuitof Fig. 5C, which is similar to the circuit of Fig. 4B except that itsportions 55 which in operation will be contiguous the beam patharepitched similarly to th more remote portions 56. However, both of thesecircuits, as the circuit of Figs. 3A and 3B, may be viewed as asuccession of'peripherally interrupted ring-like conduc-. tive members,formed by the contiguous portion of each turn or the circular portion ofthe keyhole, which are interconnected by the more remote portions ofeach turn to form a continuous wave guiding circuit.

Another alternative embodiment of the present invention is disclosed inFigs. 6A, 6B. In this embodiment, the helix-like circuit 22D comprisestwo conductive tapes 62 and 63 which arev preferably wound in the samepitch, as shown, for maximum coupling therebetween. By providing maximumcoupling between the two-tapes, it is possible to maximize thedifierence in phase velocities of the, two distinct modes capable ofpropagation along the circuit and, thus, there is facilitated theachievement of interaction with only a desired one of the two possiblemodes. Each of the tapes is wound in helix-like fashion and has an innerportion 62A, 63Av which is positioned contiguous to the beam path alongthe axis of the circuit and an outer portion 62B, 63B more'remote fromthe beam path and not in interacting relation with the beam. The twohelices are coupled in-phase to suitable external circuitry by coaxiallines 66 and 67 which correspond to lines 23 and 24, respectively, ofFig. 1. To facilitate such in-phase coupling from a single coaxial line,the two helices shown are connected together at each end. Whenalternative arrangements are used to excite the two helices in-phase, itmay be advantageous to keep the two helices separate. As in theembodiment of Fig. 3A, inner portions 62A and 63A of the instanthelix-like circuit are dimensioned to lie along an arc, each preferablysubtending an angle greater than 150, of a circle whose circumference isapproximately equal to 1.5). Thus dimension a of Fig. 6B isapproximately Additionally, as in the embodiment f Fig. 3A, such innerportions efiectively form a succession of peripherally interruptedring-like conductive members which are interconnected by outer portions62B, 63B to form a tween corresponding points on the inner portion ofsuc described are merely illustrative cessive helix-like turns measuredaround the periphery of the helix, that is, the length of one completeturn, equal to one half a free space wavelength.

It is to be understood that the specific embodiments of the general.principles of the present invention. Various other arrangements may bedevised by one skilled in the art without departing from the spirit andscope of the invention. In particular, each of the embodiments has beendescribed for use in a tube of the backward wave type. However, it is tobe understood that the principles of the invention are applicable alsoto a forward wave tube, action is of the spatial harmonic type. Such atube can be obtained merely by interchanging the input and outputterminals of the tube described. A showing of such a tube has beenomitted since it will readily appear to one skilled in the art and it isnot deemed to. be as important an application of spatial harmonicoperation as the backward wave tube.

What is claimed is:

1. A spatial harmonic travelingwave tube comprising means for forming asolid electron beam and for projecting said beam along an extended path,an electro where interfiee space wavelength, and means for establishinga finite spatial harmonic field along and substantially encompassingtheregion about the axis of said electron beam, said means comprising afirst wave propagation portion of said helix-like member contiguous tothe beam path and a second portion more remote therefrom, said firstportion defining an arc of a circle subtending an angle greater than 180degrees and whose circumference is approx-i mately equal to 1.5%, wherea is the wavelength of the wave propagating along the helix-like member.

2. A spatial harmonic traveling wave tube comprising means for formingan electron beam and for projecting said beam along an extended path, anelectromagnetic wave propagating circuit comprising a conductive tape.wound to form a helix-like structure having a distance around each turnvthereof of approximately one half a free space wavelength, andmeans forestablishing a finite spatial harmonic field along and substantiallyencompassing the region about the axis of said electron beam, said meanscomprising a first wave propagation portion of said helix-like structurecontiguous to the beam path and a second portion remote therefrom, saidfirst portion defining an arc of a circle subtending an angle greaterthan- 180' degreesand whose circumference is approximately equal to 15kwhere a is the wavelength of the wave propagating along the helix-likestructure.

3. A device which utilizes the interaction between an electron beam anda negative spatial harmonic component of. an. electromagnetic wavecomprising means for forming an electron beam and for projecting saidbeam along an. extended path, and an electromagnetic wave propagatingcircuit comprising a conductive tape wound to form a helix-likestructure having a distance around each turn thereof of. approximatelyone half a free space wavelength, each complete turn having a firstsubstantially pitchless wave propagation portion contiguous tothe beampath and a second portion remote therefrom, and pitched with respect tothe axis of said structure, and said first portion lying along the arcof a circle whose circumference is approximately 15k where A is thewavelength of a wave propagating along the helix-likestructure.

' 4. A device which utilizes the interaction between an electron beamand a negative spatial harmonic component of an electromagnetic wavecomprising means for forming an electron beam and for projecting saidbeam along an extended path, and an electromagnetic Wave propagatingcircuit comprising a conductive tape wound to form a helix-likestructure having a distance around eachturn thereof of approximately onehalf a free space wavelength, each complete t urn having a first wavepropagation portion contiguous to the beam path and a second portionremote therefrom, the contiguous and remote portions being similarlypitched with respect to the axis of the structure, and said firstportion lying along the arc of a circle whose circumferenceisapproximately 15k where k is the wavelength of a wave propagating alongthe helix-like structure.

. 5. In a device which utilizes the interaction between an electron beamand an electromagnetic wave, means for forming and projecting anelectron beam along an extended path, and a transmission line forpropagating an electromagnetic wave in coupling relation with said beamover a distance of several wavelengths at the mid-band operatingfrequency including a helix-like structure which comprises a series ofsubstantially pitchless ring-like conductive wave propagation portionsspaced apart in a linear array, each of which has at least oneinterruption point along its periphery, and a plurality of conductiveportions each being pitched with respect to the axis of the helix-likestructure and connected between the free ends of adjacent ring-likeportions to form a single conductive.- wave propagation path along acontinuous structure.

6. The combination of elements set forth in claim 5 wherein the radiusof the ring-like conductive portions; is approximately equal to and suchring-like portions are more proximate to the electron beam than theintermediate portions.

7. The combination of elements set forth in claim 5 wherein the distancebetween corresponding points on adjacent ring-like conductors measuredaround the periphery of the helix is approximately one half a free spacewavelength at the mid-band operating frequency.

8. In a device which utilizes the interaction between an electron beamand an electromagnetic wave, means for forming and projecting anelectron beam along an extended path, and a transmission line forpropagating an electromagnetic wave in coupling relation with said beamover a distance of several wavelengths at the mid band operatingfrequency including a helix-like structure which comprises a series ofpitchless ring-like conductive portions spaced apart in linear array,each of which has only one interruption point along its periphery, and aplurality of conductive portions each being pitched with respect to theaxis of the helix-like structure and being connected between theinterruption points on adjacent ring-like portions to form a singleconductive wave propagation path along a continuous structure.

9. The combination of elements set forth in claim 8 wherein the radiumof the ring-like conductive portions is approximately and the distancebetween corresponding points on successive ring-like portions measuredaround the periphery of the helix is one half a free space wavelength atthe mid-band operating frequency.

10. In a device which utilizes the interaction between anelectromagnetic wave and an electron beam for amplifying the wave, meansfor forming an electron beam and for projecting said beam along anextended path, and a transmission line for propagating anelectromagnetic wave therealong in coupling relation to said beamcomprising a conductor wound in a helix-like fashion to form aniterative pattern, each section of which includes in intermediateconductive wave propagation portion surrounding'the beam path at a zeropitch angle therewith bounded by two conductive portions extending at apredetermined pitch angle for interconnecting the-zero-pitch portions,said zero-pitch portions being in the form of a conductive ring which isinterrupted over a portion of its periphery and having its free endsthus formed connected to the pitched portions for forming a singleconductive wave propagation path.

11. In a device which utilizes the interaction between an electron beamand the spatial harmonic component of an electromagnetic wave, means forforming an electron beam and for projecting said beam along an ex tendedpath, and a spatial harmonic propagating circuit in coupling proximitywith said beam comprising a conductive member wound in helix-likefashion but differing from' a simple helix in that its cross section iskeyholeshaped rather than circular, the length of each complete turn ofsaid helix-like circuit being approximately one half a free spaceWavelength, each complete turn comleastiiOO degrees of a circle whosecircumference is approximately equal to 1.5x where 1 is the wavelengthof the wave propagating along the helix-like circuit.

13. The combination of elements set forth in claim 11 wherein thecircular portion of the helix-like circuit is pitchless and the moreremote portion is pitched at an angle with respect to the beam path.

14. The combination of elements set forth in claim 11 wherein thecircular contiguous portion of the helix-like circuit and the moreremote portion thereof are similarly pitched with respect to the beampath.

15. In a device which utilizes the interaction between an electron beamand an electromagnetic wave, means for forming and projecting anelectron beam along an extended path, and a transmission line forpropagating an electromagnetic wave in coupling relation with said beamover a distance of several wavelengths at the midband operatingfrequency including a helix-like structure which comprises a series ofpitchless ring-like conductive wave propagation portions spaced apart ina linear array, each of which has two interruption points along itsperiphery which is defined by the arc of a circle having a radius ofapproximately and a plurality of conductive portions each being pitchedwith respect to the axis of the helix-like structure and being connectedbetween adjacent ring-like portions to form a continuous structure withthe distance between corresponding points on successive ring-likeportions to form a continuous structure with the distance betweencorresponding points on successive ring-like portions measured aroundthe periphery of the helix equaling one half a free space wavelength atthe mid-band operating frequency.

References Cited in the file of this patent UNITED STATES PATENTS Re.21,739 Llewellyn Mar. 4, 1941 2,647,219 Touraton et al July 28, 19532,702,370 Lerbs Feb. 15, 1955 2,801,359 Hollenberg July 30, 19572,802,135 Dodds Aug. 6, 1957 2,853,644 Field Sept. 23, 1958 2,882,440Mourier Apr. 14, 1959 FOREIGN PATENTS 1,119,661 France Apr. 9, 1956

